diff --git a/src/ra.hrl b/src/ra.hrl
index 14503d48..2d6856f5 100644
--- a/src/ra.hrl
+++ b/src/ra.hrl
@@ -253,6 +253,7 @@
 -define(C_RA_SRV_AER_RECEIVED_FOLLOWER_EMPTY, ?C_RA_LOG_RESERVED + 17).
 -define(C_RA_SRV_TERM_AND_VOTED_FOR_UPDATES, ?C_RA_LOG_RESERVED + 18).
 -define(C_RA_SRV_LOCAL_QUERIES, ?C_RA_LOG_RESERVED + 19).
+-define(C_RA_SRV_INVALID_REPLY_MODE_COMMANDS, ?C_RA_LOG_RESERVED + 20).
 
 
 -define(RA_SRV_COUNTER_FIELDS,
@@ -294,7 +295,9 @@
          {term_and_voted_for_updates, ?C_RA_SRV_TERM_AND_VOTED_FOR_UPDATES, counter,
           "Total number of updates of term and voted for"},
          {local_queries, ?C_RA_SRV_LOCAL_QUERIES, counter,
-          "Total number of local queries"}
+          "Total number of local queries"},
+         {invalid_reply_mode_commands, ?C_RA_SRV_INVALID_REPLY_MODE_COMMANDS, counter,
+          "Total number of commands received with an invalid reply-mode"}
          ]).
 
 -define(RA_COUNTER_FIELDS, ?RA_LOG_COUNTER_FIELDS ++ ?RA_SRV_COUNTER_FIELDS).
diff --git a/src/ra_server_proc.erl b/src/ra_server_proc.erl
index 90825689..72de91d1 100644
--- a/src/ra_server_proc.erl
+++ b/src/ra_server_proc.erl
@@ -353,33 +353,57 @@ leader(EventType, {local_call, Msg}, State) ->
 leader(EventType, {leader_cast, Msg}, State) ->
     leader(EventType, Msg, State);
 leader(EventType, {command, normal, {CmdType, Data, ReplyMode}},
-       #state{server_state = ServerState0} = State0) ->
-    %% normal priority commands are written immediately
-    Cmd = make_command(CmdType, EventType, Data, ReplyMode),
-    {leader, ServerState, Effects} =
-        ra_server:handle_leader({command, Cmd}, ServerState0),
-    {State, Actions} =
-        ?HANDLE_EFFECTS(Effects, EventType,
-                        State0#state{server_state = ServerState}),
-    {keep_state, State, Actions};
+       #state{conf = Conf, server_state = ServerState0} = State0) ->
+    case validate_reply_mode(ReplyMode) of
+        ok ->
+            %% normal priority commands are written immediately
+            Cmd = make_command(CmdType, EventType, Data, ReplyMode),
+            {leader, ServerState, Effects} =
+                ra_server:handle_leader({command, Cmd}, ServerState0),
+            {State, Actions} =
+                ?HANDLE_EFFECTS(Effects, EventType,
+                                State0#state{server_state = ServerState}),
+            {keep_state, State, Actions};
+        Error ->
+            ok = incr_counter(Conf, ?C_RA_SRV_INVALID_REPLY_MODE_COMMANDS, 1),
+            case EventType of
+                {call, From} ->
+                    {keep_state, State0, [{reply, From, Error}]};
+                _ ->
+                    {keep_state, State0, []}
+            end
+    end;
 leader(EventType, {command, low, {CmdType, Data, ReplyMode}},
-       #state{delayed_commands = Delayed} = State0) ->
-    %% cache the low priority command until the flush_commands message arrives
-
-    Cmd = make_command(CmdType, EventType, Data, ReplyMode),
-    %% if there are no prior delayed commands
-    %% (and thus no action queued to do so)
-    %% queue a state timeout to flush them
-    %% We use a cast to ourselves instead of a zero timeout as we want to
-    %% get onto the back of the erlang mailbox not just the current gen_statem
-    %% event buffer.
-    case queue:is_empty(Delayed) of
-        true ->
-            ok = gen_statem:cast(self(), flush_commands);
-        false ->
-            ok
-    end,
-    {keep_state, State0#state{delayed_commands = queue:in(Cmd, Delayed)}, []};
+       #state{conf = Conf, delayed_commands = Delayed} = State0) ->
+    case validate_reply_mode(ReplyMode) of
+        ok ->
+            %% cache the low priority command until the flush_commands message
+            %% arrives
+
+            Cmd = make_command(CmdType, EventType, Data, ReplyMode),
+            %% if there are no prior delayed commands
+            %% (and thus no action queued to do so)
+            %% queue a state timeout to flush them
+            %% We use a cast to ourselves instead of a zero timeout as we want
+            %% to get onto the back of the erlang mailbox not just the current
+            %% gen_statem event buffer.
+            case queue:is_empty(Delayed) of
+                true ->
+                    ok = gen_statem:cast(self(), flush_commands);
+                false ->
+                    ok
+            end,
+            State = State0#state{delayed_commands = queue:in(Cmd, Delayed)},
+            {keep_state, State, []};
+        Error ->
+            ok = incr_counter(Conf, ?C_RA_SRV_INVALID_REPLY_MODE_COMMANDS, 1),
+            case EventType of
+                {call, From} ->
+                    {keep_state, State0, [{reply, From, Error}]};
+                _ ->
+                    {keep_state, State0, []}
+            end
+    end;
 leader(EventType, {aux_command, Cmd}, State0) ->
     {_, ServerState, Effects} = ra_server:handle_aux(?FUNCTION_NAME, EventType,
                                                      Cmd, State0#state.server_state),
@@ -1543,6 +1567,19 @@ read_chunks_and_send_rpc(RPC0,
             Res1
     end.
 
+validate_reply_mode(after_log_append) ->
+    ok;
+validate_reply_mode(await_consensus) ->
+    ok;
+validate_reply_mode({notify, Correlation, Pid})
+  when (is_integer(Correlation) orelse is_reference(Correlation)) andalso
+       is_pid(Pid) ->
+    ok;
+validate_reply_mode(noreply) ->
+    ok;
+validate_reply_mode(ReplyMode) ->
+    {error, {invalid_reply_mode, ReplyMode}}.
+
 make_command(Type, {call, From}, Data, Mode) ->
     Ts = erlang:system_time(millisecond),
     {Type, #{from => From, ts => Ts}, Data, Mode};
diff --git a/test/ra_SUITE.erl b/test/ra_SUITE.erl
index ac67c9ff..813c4632 100644
--- a/test/ra_SUITE.erl
+++ b/test/ra_SUITE.erl
@@ -15,6 +15,10 @@
 -define(PROCESS_COMMAND_TIMEOUT, 6000).
 -define(SYS, default).
 
+%% The dialyzer catches that the given reply mode is not included in the
+%% `ra_server:command_reply_mode()' type:
+-dialyzer({nowarn_function, [process_command_with_unknown_reply_mode/1]}).
+
 all() ->
     [
      {group, tests}
@@ -29,6 +33,7 @@ all_tests() ->
      start_servers,
      server_recovery,
      process_command,
+     process_command_with_unknown_reply_mode,
      pipeline_command,
      pipeline_command_reject,
      pipeline_command_2_forwards_to_leader,
@@ -300,7 +305,19 @@ process_command(Config) ->
     [A, _B, _C] = Cluster =
         start_local_cluster(3, ?config(test_name, Config),
                             {simple, fun erlang:'+'/2, 9}),
-        {ok, 14, _Leader} = ra:process_command(A, 5, ?PROCESS_COMMAND_TIMEOUT),
+    {ok, 14, _Leader} = ra:process_command(A, 5, ?PROCESS_COMMAND_TIMEOUT),
+    terminate_cluster(Cluster).
+
+process_command_with_unknown_reply_mode(Config) ->
+    [A, _B, _C] = Cluster =
+        start_local_cluster(3, ?config(test_name, Config),
+                            {simple, fun erlang:'+'/2, 9}),
+    Command = 5,
+    ReplyMode = bad_reply_mode,
+    RaCommand = {'$usr', Command, ReplyMode},
+    ?assertEqual({error, {invalid_reply_mode, ReplyMode}},
+                 ra_server_proc:command(A, RaCommand,
+                                        ?PROCESS_COMMAND_TIMEOUT)),
     terminate_cluster(Cluster).
 
 pipeline_command(Config) ->
@@ -842,7 +859,7 @@ wait_for_gen_statem_status(Ref, ExpectedStatus, Timeout)
     case get_gen_statem_status(Ref) of
         ExpectedStatus ->
             ok;
-        OtherStatus when Timeout >= 0 ->
+        _OtherStatus when Timeout >= 0 ->
             timer:sleep(500),
             wait_for_gen_statem_status(Ref, ExpectedStatus, Timeout - 500);
         OtherStatus ->